Perhaps a method that consumes general iterators would be worthwhile, e.g.

🍝(x, ::Union{HasLength,HasShape}) = String(copyto!(StringMemory(length(x)), x))
🍝(x, ::IsInfinite) = throw(MethodError(String, (x,)))
function 🍝(x, ::SizeUnknown)
    io = IOBuffer()
    for c in x
        print(io, convert(AbstractChar, c))
    end
    return String(take!(io))
end

String(itr) = 🍝(itr, IteratorSize(itr))

That would allow supporting things like String(Iterators.take("Hello, world", 5)) as well.

Attention: length is not correct in case of multi-byte characters.

Maybe Base.String(itr) = String(collect(itr)) would be sufficient (plus tests for all cases implied above)
EDIT: Not true, additionally must avoid stack overflow for e.g. String(19).

String(itr) = 🍝(itr, IteratorSize(itr))
🍝(x, ::IsInfinite) = throw(MethodError(String, (x,)))
🍝(x, ::IteratorSize) = String(collect(itr)::AbstractVector{<:AbstractChar})

Attention: length is not correct in case of multi-byte characters.

The length call in my suggested implementation is the length of the iterator with known length, not the length of a string. It's just used for allocating the memory required, and is actually identical to the implementation of String(::Memory) (IIRC).

@ararslan you may want to look at the following output to see what I mean:

julia> String( c for c in "ÄÖao")
"\xc4\xd6ao"

Ahhh yeah, thanks for the example, I see now. IteratorSize is probably not so useful a query then since HasLength only tells you that length can be assumed to be implemented. Something like

String(itr) = String([convert(AbstractChar, c) for c in itr])

should ensure you don't get a stack overflow on String(19), though the error you do get is rather opaque (MethodError on String(::Array{Char,0})).

String(x) = String_iterator(x, IteratorSize(x))
String_iterator(x, ::IsInfinite) = throw(MethodError(String, (x,)))
String_iterator(x, ::IteratorSize) = begin
    collected = collect(x)
    if !(isa(collected, AbstractVector) && all(x -> isa(x, Char), collected))
        throw(MethodError(String, (x,)))
    end
    return String(collected::AbstractVector{<:AbstractChar})
end

could something like this work? shows method error on String(19), works for all other cases provided :)